Geiger-mueller tube



Sept. 23, 1947.

COLNTING RATE E. DER MATEOSIAN EI'AL 2,427,663

GEIGER-MUELILER TUBE Filed NOV. 8, 1945 ETHYL ALCOHOL VAPOR AND ARGONETHYLENE BROMIDE AND ARGON I I I l l I I I3 l4 l5 l6 l7 l8 I9 2000gwucmlow APPLIED VOLTAGE EDWARD DER MATEOSIAN MICHAEL MC KEOWN GuamPatented Sept. 23, 1947 GEIGER-MUELLER TUBE Edward der Mateosian andMichael McKeown, Washington, D. C.

Application November 8, 1945, Serial No. 627,514

3 Claims. (Cl. 250-83.6)

(Granted under the act of March 3, 1883, as amended April 30, 1928; 3700. G. 757) Our invention relates to radiation detecting devices of thedischarge type and particularly it relates to an improved form ofGeiger-Mueller counter for the detection and measurement of varioustypes of penetrating radiations, such as a, p, and Y rays and X-rays.

The general classification of Geiger-Mueller countersinto slow and fastcounters is a well chosen one, for, as long as the cylinder andconcentric wire configuration of electrodes is maintained, the action ofa well-made counter tube in its capacity for resolving pulses is mostsensitive to the gases which it contains. The presence of an organicvapor in the gaseous filling of the tube modifies the discharge processin such a fashion that the tube recovers more readily from itsconducting state than it does when it is filled with a pure inert gas.The Geiger-Mueller counter comprising a cylindrical cathode, a coaxialwire anode, and a glass envelope enclosing the electrodes and a rare gasfilling including alcohol vapor as a quenching agent to speed thecounting action of the tube is quite well known.

Theoretically there appears to be no justification for the prevalent useof alcohol vapor as the quenching agent in Geiger-Mueller counters, forone of the decomposition products of alcohol, water, is decidedlydetrimental to the action of the tube. Unless very carefully prepared,the tubes containing alcohol vapor also tend to spark in the plateauregions of their characteristic curves.

It is an object of our invention to provide a Geiger-Mueller countercontaining a gaseous atmosphere which obviates the operatingdisadvantages of the conventional alcohol vapor quenched Geiger-Muellercounter.

It is another object of our invention to provide a Geiger-Muellercounter which is characterized by its long life, capacity to count athigh rates as indicated by its resolving power, and the long plateau ofits operating curve.

It is another object of our invention to provide a gaseous filling for aGeiger-Mueller counter which is not critically sensitive to thepreparation of the envelope and electrode surfaces and can be used in atube with assurance of the production of a reliable operable countertube.

Other objects and advantages of our invention will in part be obviousand in part appear hereinafter.

The nature, scope and advantages of our invention will be more readilyand fully understood by reference to the following specification anddrawings in which:

Figure 1 represents a longitudinal cross-sectional sketch of a typicalform of Geiger-Mueller counter;

Figure 2 is a pilot of performance curves to compare the counting actionof a. counter made according to our invention with the performance of analcohol vapor quenched Geiger-Mueller counter.

Specifically, in accordance with our invention, we have found that thecounting action of a Geiger-Mueller counter which ordinarily would befilled with a mixture of argon, or other rare gas, under a pressure of 1to 50 centimeters of mercury, and alcohol vapor, under a partialpressure of 0.05 to 2 centimeters of mercury, can be much improvedbyfilling the tube with the usual gas and alcohol vapor and, in addition,a trace of methylene bromide, or by filling the tube with the rare gasand methylene bromide in combination, or methylene bromide alone. Thatis to say, we have discovered that a trace of methylene bromide vapor inthe gaseous filling of a Geiger- Mueller counter promotes and improvesthe quenching action of alcohol vapor, the methylene bromide can be usedas the quenching agent, or the methylene bromide can be used as the solegaseous constitutent of the atmosphere of the tube.

Referring to the drawing comprising Figure 1, there is illustrated inlongitudinal cross-section a Geiger-Mueller counter having the essentialstructural elements, which include an envelope H], which envelope ispermeable to the radiation to be measured, a cylindrical cathode II, anda coaxial anode l2. Electrical connections are made to the cathode andanode, respectively, and resistor l3 and battery M are merelyconventional representations to indicate that in use the anode of thetube i held at a potential above that of the cathode.

One of the most significant measures of the efficacy of the countingaction of a tube is the length of its plateau when its performance curveis plotted in terms of counting rate above background in response to asource of radiation of a given strength against anode-to-cathodevoltage. The measure of the quality of a counter is the length of theplateau which shows in such a plot. The significance of the plateau isthat the counter will respond to radiation of a given intensity bypulsing at a known rate when any voltage within the range included inthe plateau is applied across the anode and cathode. When a counter isto be used to measure the intensity of unknown radiation it should havea long fiat plateau so that the measurement will be made in terms of aknown pulsing rate when a voltage within the range of the plateau iapplied to the tube.

A seous filling for a Geiger-Mueller tube whicl gives a long plateau inthe performance curve is highly desirable because it makes almostimmaterial certain factors in the tube construction which are ordinarilyrather critical. For example, when a rare gas-alcohol vapor mixture isused as a filling for a Geiger-Mueller tube, the alcohol is such acritical quenching agent that the tube must be constructed withmeticulous care in order that the final structure will have an operativeplateau of about 100 to 200 volts. On the other hand when a quenchingagent such as methylene bromide is used, in accordance with ourinvention, it i not necessary to take extreme precautions in theconstruction of the tube to obtain a final structure having a plateau of300 or 400 volts. We have found that when elaborate precautions aretaken in preparing the cathode cylinder and anode wire, when the metalare carefully cleaned and outgassed, and other such precautions taken,and also when the tube is filled with spectroscopically pure gas andmethylene bromide a 2.000 volt plateau can be obtained. We have alsofound that when the tube is assembled with none of these precautionmath-- ylene bromide in the tube as a quenching agent will give aplateau of 200 or 300 volts whereas a similar tube constructed under thesame conditions and using alcohol vapor as a quenching agent will givelittle or no plateau and will spark very quickly.

In Figure 2 we have plotted a performance curve for a Geiger-Muellercounter having an atmosphere consisting of argon at a partial pressureof centimeters of mercury and ethyl alcohol vapor at a partial pressureof 0.5 centimeter of mercury. This tube corresponds to the standardinternally quenched tube commonly used for radiation measurements andhas a platcan in its performance curve approximately 100 volts inbreadth. On the same coordinate axes we have plotted a similarperformance curve for a similar Geiger-Mueller tube filled with anatmosphere comprising argon at a partial pressure of 10 centimeters ofmercury and methylene bromide at a partial pressure of 0.5 centimeter ofmercury. It is apparent that the plateau in the curve for the tubecontaining methylene bromide vapor extends over about 700 or 800 volts.

The method of constructing the Geiger-Mueller tube and filling it withan atmosphere for reliable rapid quenching in accordance with our invention will be more clearly understood by rei erence to the followingdescription:

Example A single counter tube having a glass envelope, a cylindricalcathode, and a tungsten wire anode arranged as shown in Figure 1 wassealed to a vacuum pumping system, carefully evacuated, and filled witha mixture of argon and methylene bromide. A performance curve for thecounter was determined by measuring, with a scale-of- 64, the responseof the tube to a fixed source of gamma radiation as the voltage appliedto the tube was gradually increased. As can be noted from the curveshown in Figure 2 which is a, plot for a tube having a filling of 10centimeters of argon and 0.5 centimeter of methylene bromide, a. mixtureof argon and methylene bromide ex hibited very desirable counterproperties in that d a plateau '700 to 800 volts long was obtained.Another shell when filled carefully with spectroscopically pure argon at10 centimeters of mercury and methylene bromide at 0.5 centimeter ofmercury exhibited a plateau which was over 2,000 volts long and had athreshold at 1,000 volts.

The methylene bromide vapor quenched tubes are very much moreinsensitive to visible and ultraviolet light than the alcohol tubes. Thebackground count was 8 counts per minute for the argon-methylene bromidemixture and 50 per minute for the alcohol argon counter. Similar resultsare obtained when the other gases are used with the methylene bromide,namely, neon, krypton, xenon, hydrogen, helium, etc. The usable lifetimeof tubes constructed according to our invention is generally at least 5to 10 times as long as that for alcohol vapor quenched tubes used forimilar purposes.

In the construction of the tubes the materials used are not critical. Wehave obtained substantially similar results with copper, brass, nickel,aluminum, chrome iron, gold etc. cathodes.

From the above example and description it will be apparent thatmethylene bromide when used in combination with other gases or a.filling in a Geiger-Mueller counter easily improves the operation of thetube several fold. With reasonable care, the tube plateau can beimproved and the tube lile increased by a factor of at least 2 or 3.

The pressure of the rare gas such as argon and the quantity of methylenebromide to be used in a Geiger-Mueller tube is determined by theintended use of the tube and the physical limitations of thecharacteristics. The greater the proportion of methylene bromide presentin the gaseous filling of the tube, the greater can be the expectedlifetime of the tube but the higher will be its threshold voltage. Lessthan 0.05 centimeter of mercury partial pressure of methylene bromidevapor in the tube filling will give rather poor and irreproduceablecounting action of the resultant tube. The upper limit of the partialpressure of methylene bromide vapor to be used in the tube is determinedgenerally by the vapor pressure of methylene bromide at ambient temperatures. The normal amount to be used is a partial pressure of about0.1 centimeter to about 4 centimeters of mercury of methylene bromidevapor. Since methylene bromide (CHzBrz) is 8. liquid having a boilingpoint of 97 C., its vapor can be easily introduced into a tube in theamount needed.

As a general rule the threshold voltage of the tube will increase withthe quantity of rare gas present. The average for the ordinary tube willbe from 3 to 50 centimeters of rare gas such as argon. For a gamma raycounter with a low threshold voltage a gaseous filling comprising about3 centimeters of argon and 0.1 centimeter of methylene bromide vaporshould be satisfactory. This type of tube is one which depends uponimpingement of the radiation on the oathode to cause a discharge withinthe tube. Another extreme in operation is that type of tube involvingthe absorption of relatively soft radiation by the gaseous filling ofthe tube, that is, to rely upon a photoelectric effect within the gas.This form of Geiger-Mueller counter is described more fully in UnitedStates Patent 2,386,785, issued October 16, 1945, to Herbert Friedman.The tube is also described in a paper discussing the application ofGeiger counters to X-ray diffraction measurements published InElectronics, April 1945. Where the radiation to be measured is to beabsorbed by the gaseous filling of the tube, the Geiger counter would befilled with argon at a pressure of about 50 centimeters of mercury andabout 0.3, centimeter of methylene bromide. In such a counter it isdesirable to provide a gas path of sumcient length to absorb all of theradiation or a known fraction thereof. For similar applications we havealso found that a counter filled with an atmosphere consisting only ofmethylene bromide vapor gives reliable and rapid counting action.

Thus, it is apparent that in one aspect of our invention the methylenebromide vapor can be used as a quenching agent in a Geiger-Muellercounter to assist the normal quenching action of alcohol vapor in muchthe same fashion that certain promoters are used with catalysts inchemical reactions. A second aspect of the invention involves thediscovery that methylene bromide not only assists. the quenching actionof the alcohol vapor but, when used alone, it at least equals andusually far exceeds alcohol vapor as a quenching agent in eflicacy. Athird aspect of the invention deals with the discovery that methylenebromide vapor can be the sole gaseous filling of the tube.

The essential advantages derived from the use of methylene bromide in aGeiger counter are prolongation of the useful lif of the tube, extensionof the tube plateau in performance, and improved response to radiationof a given intensity.

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

Having described our invention, what we claim as new and desire tosecure by Letters Patent of the United States is:

1. A Geiger-Mueller tube characterized by its long life and rapidcounting response to penetrating radiation comprising, in combination, aradiation permeable envelope enclosing an anode and cathode, and anatmosphere containing methylene bromide vapor.

2. A Geiger-Mueller tube characterized by its long life and rapidresponse to penetrating radiation comprising, in combination, aradiation permeable envelope enclosing an anode and cathode, and anatmosphere containing methylene bromide vapor in amount suflicient tomake the partial pressure of said compound about 0.05 to about 5centimeters of mercury.

3. A Geiger-Mueller tube characterized by its long life and rapidresponse to penetrating radiation comprising, in combination, aradiation permeable envelope enclosing an anode and cathode and anatmosphere consisting of methylene bromide vapor.

EDWARD nsa MATEOSIAN. MICHAEL McKEOWN.

